1. Field of the Invention
This invention relates to a frequency converter, and more particularly, to a frequency converter suitable for a multi-channel broadcast signal receiver.
2. Description of the Prior Art
Generally, the basic function of a frequency converter is to convert frequencies of received broadcast signals into prescribed frequencies. In this frequency conversion operation, although it is necessary to take a sufficient signal-to-noise ratio (S/N) into account to obtain the required power gain, the frequency conversion operation must be performed so that a prescribed selectivity can be maintained. Also, to avoid signal distortion, such as cross-modulation distortion, the gain of the frequency converter must be controlled. However, it has been difficult to eliminate the signal distortion and improve the S/N simulteneously since an improvement in one results in a degradation of the other. Therefore, the prior art faces a problem of how to suppress the signal distortion and prevent a simultaneous deterioration of the S/N of the frequency converter.
Apart from receivers for conventional television broadcasting, this problem also affects receivers for CATV broadcasting, which is a multi-channel broadcasting with many transmission channels.
In a CATV converter which functions as a tuner-converter for a CATV receiver, channel frequencies, i.e., first-RF signal frequencies of received CATV broadcast signals, are first converted into corresponding second-RF signal frequencies which are higher than the first-RF signal frequencies of the received CATV broadcast signals by a first mixer (up conversion) and then selected by a second-RF tuned amplifier and converted into a prescribed television channel frequency representing a vacant channel (non-broadcast channel) among a general television broadcast channel band, e.g., the VHF band, or the UHF band by a second mixer (down conversion). This type of frequency converter is known as an up-down frequency converter because it first converts the first-RF signal frequency into the second-RF frequency which is higher than the first-RF signal frequency and then converts the second-RF frequency into the prescribed television channel frequency which is lower than the first-RF signal frequency.
The frequency conversion as described above is carried out on the respective multiple CATV broadcast signals, while the received CATV broadcast signals or the first-RF signals are transmitted through a coaxial cable and applied to the frequency converter. Levels or intensities of individual channel signals of the CATV broadcast channels band are not always the same. Therefore, a tilt amplification characteristic can be employed at a line repeater which is provided in a transmission line for a reception terminal, i.e., the frequency converter, to make its gain vary for the lower and higher transmission channel frequencies. The tilt amplification characteristic of the line repeater is determined by responding to level diviations among channels in the reception terminal, signal distortion in the transmission system, etc. That is to say, the transmission characteristics of the tilt amplification characteristic of the line repeater should be determined by taking into account the signal distortion and the S/N of frequency converter in the reception terminal.
FIG. 6 shows a circuit of a conventional up-down frequency converter. In the Figure, a plurality of CATV broadcast channel signals are applied to an input terminal 1. Then plurality of CATV broadcast channel signals are inputted to a first mixer 4 via a band pass filter (BPF) which comprises of a high pass filter (HPF) 2 and a low pass filter (LPF) 3. First mixer 4 also has applied an output of a first local oscillator 6 via an amplifier 5. The frequencies of the input CATV broadcast signals are raised in first mixer 4, respectively, to frequencies each higher by a first oscillation frequency of first local oscillator 6. The frequency-converted signals, i.e., second-RF signals from first mixer 4 are input to second mixer 10 via a first frequency gate which comprises a BPF 7, a second-RF signal amplifier 8 and a BPF 9. The first frequency gate passes through it a signal with a prescribed second-RF frequency of the second-RF signals to second mixer 10. Second mixer 10 receives the output of a second local oscillator 11. Second mixer 10 lowers the prescribed second-RF frequency of the signal that passed through the first frequency gate to a prescribed frequency which corresponds to a vacant channel (non-broadcast channel) among the afore-mentioned conventional television broadcast channel band, e.g., the VHF band or the UHF band, in using the oscillation output of second local oscillator 11. The frequency-converted signal is output from output terminal 13 via output BPF 12.
As described above, it is generally desirable that the frequency converter should not deteriorate its noise figure (NF) characteristics and should suppress any signal distortion occuring therein.
When a non-linear signal distortion occus in an amplifier, generally there is a following relationship between an input signal voltage and an output voltage of the amplifier. ##EQU1## wherein, Ye: Output signal voltage of amplifier
e: Input signal voltage of amplifier PA1 Kn: Coefficient presenting a linearity of amplification the amplifier PA1 n: Order of signal distortion
Although the non-linear signal distortion occurs to fairly high orders as seen from Equation (1), only the second order distortion component (n=2) and the third order distortion component (n=3) need to be considered for practical use. When the amount of the signal distortion given by Equation (1) rises in an amplifier costituting the frequency converter, a cross-modulation disturbance and a beat disturbance occur. The degree of the cross-modulation disturbance is proportional to the square of the amplitude of the signal which interferes with the desired signal. Moreover, the cross-modulation disturbance becomes greater as number of the received broadcast channel signals increases.
On the other hand, the beat disturbance occurs when signal distortions occurring for a plurality of the received broadcast channel signals are present in the television broadcast channel band. For reducing the effects of the cross-modulation distortion and the beat disturbance, pre-amplifier 14 could be removed from the circuit arrangement shown in FIG. 6. However, then the carrier-to-noise ratio (C/N) at the frequency converter worsens due to the lack of the pre-amplifier.
The C/N is generally expressed by EQU C/N[dB]=e.sub.i [dB u]-NF[dB]-0.8[dB] (2)
Also, a total amount of the C/N is given as follows, EQU C/Nm[dB]=C/N[dB]-10 log.sub.10 m[dB] (3)
wherein m represents the number of amplifier stages connected in cascade. As seen from Equation (3), the total amount of the C/N, i.e., the C/Nm, is inversely proportional to the number of amplifier stages in cascade; m. In other words, when m number of amplifier stages of the same performance are connected in cascade, the C/N of the frequency converter wersons by 10 log.sub.10 m[dB]. Therefore, when m number of amplifier stages are connected in cascade, each amplifier stage requires for its input signal a level of e.sub.i, given in the following Equation (4), in order to maintain the C/N value the same as when only one amplifier stage is used. EQU e.sub.i [dB u]=e.sub.min [dB u]+10 log.sub.10 m (4)
wherein e.sub.min represents the lowest signal input level which is obtained using Equation (2).
It is clear from Equation (4) that, in order to obtain the C/N over a prescribed value, the input signal is required to be at a sufficient level over a prescribed level.
Therefore, the input signal level for the frequency converter must be set to an optimum level 30 to satisfy both requirements of 10W signal distortions and high C/N.
In the conventional frequency converter shown in FIG. 6, since no pre-amplifier is provided prior to first mixer 4, although a lower processed signal level is desirable for reducing the signal distortion, the C/N is deteriorated since the input signal level is insufficient to satisfy the prescribed C/N required in the rear stage amplifier, e.g., second-RF amplifier 8.
For resolving the problem, a pre-amplifier is provided prior to the first mixer, for example, in a position between HPF 2 and LPF 3 which are shown in the FIG. 6. This pre-amplifier is employed at the cost of increasing the signal distortion, such as the cross-modulation distortion. FIG. 7 is a circuit diagram showing the construction of this type of circuit, and it differs from the circuit in FIG. 6 in that amplifier 14 is provided. Amplifier 14 is generally called a pre-amplifier. It is provided for preventing the deterioration of the C/N of the frequency converter.
In the frequency converter shown in FIG. 7, pre-amplifier 14 amplifies the input signal to a required level, given by Equation (4) and contributes to the obtaining of the prescribed C/N. On the other hand, the second and the third order distortions are increased.
That is to say, although the C/N is improved, cross-modulation distortion will occur if there is non-linear distortion in pre-amplifier 14. Thus it is necessary to control the gain of pre-amplifier 14 so that the signal distortion is not increased by excessive gain.
When an m number of amplifier stages are connected in cascade, if power gains of the respective amplifier stages are taken as G1, G2, . . . Gm and the NFs of the respective amplifier stages are taken as NF1, NF2, . . . NFm, the total niose figure NFt is expressed by ##EQU2##
Thus for improving the NF and the C/N, it is advantageous to heighten the gain of the amplifiers in the rear stages of the frequency converter. On the other hand, for suppressing the signal distortion, it is desriable to heighten the gain of the amplifiers in the front stages of of the frequency converter. Therefore, with respect to the gains of the amplifiers, the NF or the C/N characterstic of its frequency converter and the signal distortion characterstic have responses inconsistent with each other.
In the conventional frequency converter as shown in FIG. 7, either pre-amplifier 14 or second-RF amplifier 8 is made so that its gain may be controlled automatically in response to the output of the frequency converter. That is, an automatic gain control (AGC) is performed in one of pre-amplifier 14 or second-RF amplifier 8.
If the AGC is carried out in second-RF amplifier 8, a sufficient level of the input signal must be maintained over the level which satisfies Equation (4), so that the C/N will not be excessively deteriorated. However, signal distortion becomes severe when the level of the input signal exceeds a predetermined level in conjunction with the AGC. This is because the AGC is carried out for the signal from first mixer 4, in which the signal distortion has occured due to non-linear characteristic elements of first mixer 4 for effecting the frequency conversion.
Moreover, in the case when the AGC is carried out in pre-amplifier 14, there is a limit to the extent of gain reduction (GR) due to the AGC, because pre-amplifier 14 is, for example, a 55-450 MHz broad band amplifier. Thus the GR for pre-amplifier 14 can not be expected to sufficiently suppress the signal distortion.
Thus, in the conventional frequency converters shown in FIGS. 6 and 7, since both the C/N characteristic and the distortion characteristic are prescribed, recently there has been a problem in that it is difficult to control the signal gain for those levels.